Field of the Invention
In making decorative laminates, a plurality of sheets impregnated with a thermosetting resin are assembled in a stacked, superimposed relationship with a decorative sheet placed on top. The assembly is placed between platens and then heat-and-pressure consolidated.
Generally, more than one laminate is formed at one time by inserting a plurality of assembled sheets in a stack with each assembly being separated by a release sheet which allows the individual laminates to be separated after consolidation. The laminates so formed are then bonded to a reinforcing substrate, such as plywood, particle board or the like, by the use of adhesives such as contact adhesives, urea-formaldehyde, white glues (polyvinyl acetate emulsions) hot melts, phenolic or resorcinol-formaldehyde epoxy, coal tar, animal glues, and the like. The glues vary in their cost and reliability. A very commonly used glue in the industry is a polyvinyl acetate emulsion sold under the name Wood-Lok No. 40-0536. This glue is inexpensive and reliable, but is not versatile in the variety of laminating surfaces it will adhere to.
In the typical operation where polyvinyl acetate glues, as well as many of the other glues, are used, the laminate is first sanded on its back surface before it is glued to its supporting substrate. This is because the conventional glues will not directly adhere to the phenolic impregnated back sheet of the laminate. In addition, some of the release mediums used to separate laminates during consolidation permit some of the release material to transfer to the back surface of the laminate and reduce the gluability of the laminate to the reinforcing substrate. By back surface, it is meant the exposed flat surface of the laminate which is furthermost from the decorative sheet and which is to be glued to the reinforcing substrate.
The sanding operation is an expensive step in the process and adds to the cost of the product. The step can also be responsible for increased waste of the laminated material. Since the laminates are relatively inflexible after the resins contained therein have been thermoset during the consolidation process, the thin laminates are particularly susceptible to damage during the sanding step due to their brittleness.
It may be possible to eliminate the sanding step by using certain glues which have the ability to adhere to the phenolic impregnated back sheet. However, such glues are expensive and have not been readily accepted by manufacturers of reinforced laminates. Each manufacturer has learned through years of experience that he can rely upon certain glues. When considering his potential liability for laminates that separate from their reinforcing substrate after they have been fabricated into expensive furniture, cabinets and the like, it is understandable why the manufacturers are reluctant to change glues. Accordingly, the prevailing practice in the industry continues to be sanding the back surface of the laminate and using the well tested polyvinyl acetate glues.
There are existing techniques which are designed to provide a gluable back surface for the laminate without sanding. For example, U.S. Pat. No. Re. 27, 644 to Kelly and Grosheim discloses a laminate coated on its back surface with a polyvinyl acetate composition. However, laminates produced in accordance with this technique have the disadvantage of not being easily consolidated in contact with one another in a stack without adhering to one another during the consolidation process.
U.S. Patent Application No. 600,225, filed July 30, 1975 in the names of William A. Hosmer and William M. Bowler, discloses a gluable backing sheet which becomes the back surface of a decorative laminate during consolidation and provides a surface which is gluable to reinforcing substrates with a variety of the most commonly used glues. In addition, the Hosmer/Bowler invention provides the ability to consolidate laminates in back-to-back arrangement by providing a pair of backing sheets placed together between laminates, with each backing sheet ahdering to a different laminate. The backing sheet is coated with a composition of an inert filler (such as clay), a non-thermoplastic adhesive (such as proteinaceous adhesives) and a water-soluble, thermosetting resin selected from the class consisting of urea-formaldehyde resin condensates and melamine-formaldehyde resin condensates. A disadvantage of the Hosmer/Bowler invention is that two separate coated backing sheets are required when consolidating laminates in back-to-back configuration.
U.S. Pat. No. 4,030,955 to Antonio and Uhl, discloses a transferable, gluable coating which adheres to the back surfaces of the decorative laminates during the consolidation step and provides a gluable surface which can be adhered to a reinforcing substrate without sanding the back surface of the laminate. The gluable layer is first coated on both sides of a release film of polypropylene and placed between back-to-back laminate assemblies. The coating then transfers to the laminate during consolidation. The gluable coating material employed is chosen from polyvinyl alcohol, animal glue and starch. A disadvantage of this invention is that the gluable coatings must be transferred from only particular release mediums, such as polypropylene sheets. If the gluable coating of Antonio/Uhl is transferred from a paper sheet coated with a release material, which is a particularly satisfactory and inexpensive release medium for separating laminates during consolidation, the gluability of the coating can be seriously impaired.